1. Field
One embodiment of the present invention relates to a patterned medium and a method of manufacturing the patterned medium.
2. Description of the Related Art
In recent years, patterned medium has been studied as a technique for improving the recording density of hard disk drives (HDD). The patterned medium includes a discrete track medium (DTM) and a bit patterned medium (BPM). The DTM comprises patterns obtained by processing a magnetic recording layer in units of recording tracks. This serves to suppress the interference between the adjacent tracks, thus improving recording density. The BPM comprises patterns obtained by processing a magnetic recording layer in units of recording bits. This serves to increase the volume of a magnetization reversal unit compared to that of conventional media to suppress a thermal fluctuation phenomenon, thus improving the recording density.
Both the DTM and BPM need to be recordable on both surfaces of the medium, like current magnetic recording media, in order to add a high value to a drive. To achieve this, a magnetic recording layer needs to be processed on both surfaces of the medium.
The DTM or BPM is manufactured as follows, for example. First, a Si substrate is coated with a resist layer to which fine patterns are written by electron beam writing. The resist is developed to produce a master plate with patterns of recesses and protrusions of the resist layer. A Ni film is sputter-deposited on the master plate, and a Ni plated layer is deposited on the Ni film. The Ni film and the Ni plating layer are peeled off to produce a Ni stamper. The Ni stamper is set to an injection molding machine. The resin is molded using the Ni stamper as a mold to produce a replica of the stamper made of the resin (resin stamper). A magnetic recording layer is deposited on both surfaces of a medium substrate by a general method, and an ultraviolet (UV) curable resin layer is coated to both surfaces of the medium substrate. The resin stampers are pressed against both surfaces of the UV resin layer on the medium. The UV curable resin is then irradiated with UV through the resin stampers on both surfaces to be cured. The resin stampers are then peeled off to form etching mask patterns of the UV curable resin on both surfaces of the medium. The magnetic recording layer is etched by reactive ion etching (RIE) or ion milling through the patterns of the UV curable resin as etching masks. The recesses in the etched magnetic recording layer are filled with a nonmagnetic material, as needed, to flatten the surface of the magnetic recording layer as required. An overcoat layer is deposited on the magnetic recording layer, and a lubricant is coated to the overcoat layer to manufacture a DTM or BPM.
The inventors studied two-side processing of patterned medium based on the above-described process, and encountered a phenomenon that an error rate is locally degraded on both surfaces of the medium. The local degradation in error rate was also found in a patterned medium with only one surface processed, but it was due to the defect of shape of grooves/bits in the stamper or process damage. However, the local degradation in error rate shown in the two-side processed medium showed a tendency different from that caused by the defect of track/bit shape or process damage. Thus, the inventors assumed that the local degradation in error rate is inherent in the patterned medium with both surfaces processed and is caused by deviation between the center of the medium substrate and the center of the stamper.
Jpn. Pat. Appln. KOKAI Publication No. 2004-192733 discloses a method of evaluating an eccentric discrete medium without the need for tracking servo control. Jpn. Pat. Appln. KOKAI Publication No. 2004-192733 discloses that a discrete track medium is formed such that the center of tracks deviates from the center of the medium and that also when the discrete track medium is installed in an evaluation apparatus, the center of the tracks deviates from the center of rotation of the motor. The particular evaluation technique takes the eccentricity of the discrete medium into account, but does not prevent the local degradation in error rate resulting from the two-side processing of the medium as described above.